This invention relates to an optical imaging method for use with indentation instruments.
Indentation instruments use a mechanical probe tip which is typically spherical, conical, or pyramidal in shape, to determine certain mechanical properties of a sample. In traditional indentation, the probe tip is typically forced into the sample using a known force, and the size of the resulting indentation is used to determine the hardness of the sample. In more advanced indentation instruments, the indentation depth is recorded along with the load force. This is done by attaching the tip to a transducer capable of measuring and/or generating force and displacement This is commonly referred to as “instrumented indentation”. This technique allows determination of the elastic modulus as well as the hardness. When operating these instruments it is often highly desired if not necessary to align the probe tip to the sample with a high level of precision. This is commonly done using an optical microscope, but since the probe tip, as well as the load and/or depth sensing apparatus for the indentation application blocks the optical path of the microscope, inconvenient means must be resorted to in order to perform the alignment.
For large indentation instruments this is typically accomplished by using a mechanical stage to move the sample back and forth between an optical microscope and the indentation apparatus. Large stages with the required precision to locate a feature on the sample using the microscope and then translate that feature to align with the probe tip with a position error on the order of a micron adds considerable expense to the system, and the added size tends to degrade mechanical stability. In smaller systems the microscope may be tilted so as to avoid the blocking effects by viewing the tip and sample from the side. This avoids the need for the large stage as the sample does not need to be moved between the microscope and probe tip, but viewing the features on the sample that the probe tip is being aligned to is more difficult, since the tilted microscope only has a narrow band of the sample in focus at once.
It would be highly desirable to be able to view the sample surface from directly above, and align a feature on the sample to the probe tip without having to translate the sample back and forth between the optical microscope and the tip, but with current equipment this is not possible. Most currently available transducers and tips are too large to fit between the sample and a microscope objective, and even if they could, the transducers are constructed such that there is no way for light to pass through them. Indenter probe tips are also too large, both in length and diameter, so that they do not physically fit in the available space due to the height, and the excessive diameter blocks the light path of any available microscope objective.